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Amorphous magnetic state established by Mössbauer spectroscopy on powders obtained by borohydride reduction
Journal of Magnetism and Magnetic Materials 62 (1986) 87-92 North-Holland, Amsterdam
87
AMORPHOUS MAGNETIC STATE ESTABLISHED BY MOSSBAUER SPECTROSCOPY ON POWDERS OBTAINED BY BOROHYDRIDE REDUCTION Dimiter BUCHKOV, Serafim NIKOLOV *, Iovka DRAGIEVA and Mina SLAVCHEVA Higher Institute of Mechanical and Electrical Engineering * Higher Chemical Technological Institute, Sofia, Bulgaria Received 21 October 1985; in revised form 13 June 1986
The MiSssbauer spectra of a series of iron-cobalt-boron powders with different boron contents are discussed in association with the conditions of borohydride reduction. The most important structural pecufiarities due to the composition and the amorphous-crystalline state of the powders are established on the basis of the hyperfine field distribution.
1. Introduction Considerable attention has been paid lately to the peculiarities of fine metal and alloy powders prepared by reduction of dissolved metal salts with sodium borohydride. A series of papers [1-4] deals with the magnetic properties, particle size and shape, and corrosion resistance of these powders as well as with their dispersion and application in the magnetic recording media. The incorporation of boron as a result of the reduction process necessitates registering of chemical and structural short-range order in the microcrystalline and amorphous states of the powders. Our X-ray studies [5] showed these powders to be amorphous. We investigated further [6,7] the principal technological parameters of the reduction process, i.e. reducing agent concentration, kind and amount of the complex-forming agent, effect on the amount of boron incorporated in the powders of the addition of a second and a third metal to the main one belonging to the iron-nickel-cobalt group. Data concerning the particle size and shape and the effect of the magnetic field during the process on the texturing of the interglobular space are presented in ref. [8]. Precise investigations [9-12,6] on the phase composition and amorphous state of systems with a similar composition obtained by other methods demonstrated the specificity and possibilities of
MtSssbauer spectroscopy. A new method was proposed [13] for the estimation of the internal hyperfine magnetic field distribution P(H). The purpose of the present paper was to study, by this method, the disordered microcrystalline and amorphous states of powders obtained by borohydride reduction.
2. Sample preparation and experimental procedines The powder samples with the general composition (Fe,Co)l_xB~ were prepared by chemical reduction of aqueous solutions of metal salts with NaBH 4, different kinds and amounts of complex forming agents being used, the protection against oxidation being the same for all samples. Table 1 shows the initial concentrations of the reducing and complex-forming agents as well as the boron contents of the samples as determined quantitatively by a known method [14]. The powders were pelleted using polyvinyl alcohol as a binder and were pressed under a low pressure to be made suitable for MSssbauer studies. The thickness of the absorber with respect to the iron content was 20 mg/cm 2. The spectra were recorded according to a standard scheme of a MSssbauer spectrometer functioning with a constant acceleration, with a multichannel analyzer
0304-8853/86/$03.50 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
88
D. Buchkov et a L / M'6ssbauer spectroscopy on iron- cobalt-boron powders
Table 1 Technological conditions of the preparation of powdery samples by borohydride reduction Sample
Complexforming agent
Complexforming agent concentration (mol/I)
Reducing agent (NaBH 4) concentration (mol/1)
Boron content in the sample (wt~)
A
citric acid citric acid citric acid lactic acid lactic acid potassiumsodium tartrate
0.010
0.25
1.74
8.38
0.021
0.25
1.52
7.37
0.031
0.25
0.87
4.34
0.0690
0.062
2.51
11.48
0.0690
0.079
3.02
13.86
0.0035
0.189
5.25
22.27
0.069 0.0684
0.25
3.34
15.15
B C D E F
K
lactic acid glycerol
and a 57Co source on a palladium substrate. The width of the outer lines of a standard M/Sssbauer a-Fe thin absorber was below 0.3 mm/s. The spectra were obtained at room temperature (RT, 295 K) and at liquid nitrogen temperature (LNT, 77 K) and are presented with respect to an a-Fe MiSssbauer absorber. The spectrum of sample K immediately after its preparation as well as the spectrum K* of the sample upon its crystallization in vacuo at 10 -6 Torr and a temperature of 650 K for half an hour and subsequent pelleting are given in fig 3.
3. Results and discussion
The main technological conditions of preparation of the powders, which determine the composition and the structure of the final product, are the rate-determining factors such as concentration of reducing and complex-forming agents and ratio between the metal ions in the solution (table 1). The alteration of these parameters (the other technological conditions being constant) results in substantial changes in amount and arrangement of
Boron content in the sample (ate)
the iron, cobalt and boron atoms, which is evident from the MSssbauer spectra given in figs. 1 and 2. Fig. 1 presents the spectra of a series of samples (A, B, C) prepared with varying concentration of the complex-forming agent. The effect of the latter consists in a decrease in the rate of the process of powder formation with a increasing concentration of the complex-forming agent. The spectra of samples D, E and F given in fig. 2 show the effect of the different boron contents obtained by varying the reducing agent amounts and using different kinds of complex forming agents. The iron/cobalt ratio in all samples is 5.7:1. Fig. 3 shows the spectra of sample K as well as the MSssbauer spectrum of the same sample after its crystaniTation (spectrum K*). For all the samples investigated the spectra look the same at room temperature and nitrogen temperature. Therefore, the broadening of the lines is not due to superparamagnetic phenomena in the samples [15]. The spectrum K* of the crystallized sample shows narrower lines, which unambiguously indicates the transition of sample K from the amorphous to the crystalline state.
89
D. Buchkov et al. / M'6ssbauer spectroscopy on i r o n - cobalt-boron powders
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The spectra of all samples have two principal components: a sextuplet produced by a predominating ferromagnetic fraction in the sample and a paramagnetic doublet in the central part of the spectrum. The intensity ratio of these two comPonents varies depending on the preparation conditions of the samples. On the basis of the measured values of the isomeric shift, 8E = 0.4 m m / S and the quadrupole splitting 8 -- 0.5 m m / s , this superparamagnetic phase corresponds to an oxide phase containing Fe 3+. For K and K* sampies this oxide phase is below 2%. The properties of the samples greatly depend on the magnetic fraction which corresponds to the
Fig. 2. M ~ s b a u e r spectra at room temperature (RT) of samples D, E, F with different boron contents due to different NaBH 4 concentrations.
sextuplet component of the spectrum. This component is obtained as a result of overlapping of a large number of elementary sextets, each of them depending on a different value of the hyperfine magnetic field H at the iron nucleus. Due to the chemical and topological disorder of the shortrange surrounding, the hyperfine magnetic field with respect to the different iron atoms can be considerated as a random quantity characterized by a density of distribution P ( H ) . For the sake of brevity, the latter is called hyperfine field distribution. The other M/Sssbauer parameters (quadrupole splitting and isomer shift) in the case of amorphous and disordered ferromagnetic alloys have a considerably weaker effect on the shape of the spectrum. They cause an additional broadening of the lines and eventually the appearance
90
D. Buchkov et al. / M~ssbauer spectroscopy on i r o n - c o b a l t - b o r o n powders
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of a slight spectrum asymmetry. As a first approximation, the latter factors can be neglected, especially in the case of a more general discussion of the magnetic structure and the P ( H ) distribution• It is difficult and not always possible to achieve a good accuracy in the determination of P ( H ) from the experimentally recorded spectrum. However, the approximate shape and principal parameters of I ' ( H ) can be obtained by different methods [16]. A new method [13] of computer decom-
position of the symmetric spectrum is used• This method can easily be adapted to the more complicated case of a paramagnetic component in the central part of the spectrum by truncation of calculating to this region. In addition, the slight assymetry due to a low (different from zero) mean value of the quadrupole splitting affecting the magnetic fraction is taken into account• For this purpose, the quadrupole shift of the four inner sextet lines during decomposition is pre-set. The final P ( H ) distribution is obtained after a special deconvolution procedure [17] of the resultant one-component spectrum eliminating the Lorentzian broadening of the lines• T h e P ( H ) distributions presented in fig. 4 for the three most illustrative cases indicate the different types of local short-range order in the powders under investigation• For the sake of convenience, the arrows in the upper part of this figure show the positions of the hyperfine magnetic field for crystalline alloys with analogous compositions obtained by different authors [9,12]• The Roman numerals I, II and III denote the H values given in the cited papers for the different crystallographic non-equivalent positions of the iron atoms which are typical of tetragonal structures, isostructures of the type FeaP (three positions) and ortho, rhombic structures of the type Ni3P (two positions) [12]. In the all three experimental cases presented in fig. 4, the right-hand side of the distribution shows single peak of Fe-Co metal phase with no boron among the nearest neighbours of iron. This phase exhibits the highest intensity (about 41%; of the magnetic fraction) for the crystallized sample K* and the lowest intensity (about 9%) for sample K. The hyperfine magnetic field obtained for this component is (362 + 3)kOe, which in the case of FessCo]5 is in good agreement wi~ the data reported for the system Fe-Co [10]. The hyperfine field distributions in fig. 4 demonstrate the quantitative changes in phase composition due to crystallization. The powders obtained at a high reduction rate display a disorder of the main component atoms (iron, cobalt, boron)• Upon heat-treatment, concentration gradients of the components appear within the limits of the separate particles. As a result, the amount of a
D. Buchkov et aL / M'6ssbauer spectroscopy on iron-cobalt-boron powders I
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termination of the dominating form during crystallization is difficult. However, on the basis of the P ( H ) distribution (fig. 4, sample K) one may conclude that the ordering in the short-range surrounding of the iron atoms is not completely arbitrary, contrary to the case of the dense-random-packing model. Formations with 2,3 and 4 boron atoms as near neighbours of iron are prevailing according to the chemically correlated disorder model proposed by Vincze et al. [9] for F e - B metallic #asses. The P ( H ) distribution of sample B (fig. 4) illustrates the decrease in the rate of the powder synthesis due to the effect of certain complex-forming agent. The amount of the metal phase (crystalline, bcc F e - C o ) is considerably larger, but the total boron amount in this sample is smaller. In this case, formation of F e - B phases is observed on which type I-positions dominates over type II. This non-uniform population is not possible when crystal phases of the type FeaB are formed. Hence, during borohydride reduction process, the iron-boron phases formation begins as in the amorphous state.
References
400
t
HC,Oe3 Fig. 4. Hyperfine magnetic field distribution P(H) obtained from the spectra of samples K*, B and K.
crystalline bcc F e - C o phase increases. In powders not subjected to a heat treatment the presence of boron causes a large disorder in the short-range surrounding which cannot be interpreted as a simple superposition of microcrystalline formations, i.e. this is a ease of topological disorder characteristic of the amorphous state. Crystallites of the types (FeCo)3B and (FeCo)2B appear during crystallization. Due to overlapping of the lines for (FeCo)2B with those for Fe m- and Fen-positions in (FeCo)3B in the cases of tetrahedral and orthorhombic forms, respectively, a quantitative de-
[1] A. Watanabe, T. Uechori, S. Saitoh and I. Imaoka, IEEE Trans. Magn. MAG-17 (1981) 1455. [2] A. Baltz, W.O. Freitag, A.P. G-reiferand V. Suchodolski, J. Appl. Phys. 52 (1981) 2456. [3] A. Baltz and W. Doyle, J. Appl. Phys. 50 (1979) 780. [4] Y. Taketomi, A. Watanabe, Y. Tokuoka, H. Sugihara and Y. Imaoka, J. Magn. Magn. Mat. 31-34 (1983) 905. [5] I. Dragieva, G. Gawrilov, D. Buchkov and M. Slavcheva, J. Less-CommonMetals 67 (1979) 375. [6] D. Buchkov, S. Nikolov, I. Drag/eva and M. Slavcheva, Yubileina nauchna sesiya"20 godini VMEI-Varna",Varna (1983). [7] I. Dragieva, M. Slavchevaand D. Buchkov, J. Less-Common Metals 117 (1986) 311. [8] D. Mehandjiev, I. Dragieva and M. Slavcheva, J. Magn. Magn. Mat. 50 (1985) 205. [9] I. Vincze, T. Kem6ny and S. Arajs, Phys. Rev. B 21 (1980) 937. [10] I. Vincze, I.A. Campbell and A.J. Meyer, Solid State Commun. 15 (1974) 1495. [11] J.M. Dubois and G. Le Caer, Nucl. Instr. and Meth. 199 (1982) 307. [12] M. Wojcik, H. Lerchner, P. Deppe, F.S. Li and M. Rosenberg, J. Appl. Phys. 55 (1984) 2288.
92
D. Buchkov et al. / M~ssbauer spectroscopy on iron- cobalt-boron powders
[13] S. Nikolov, Solid State Commun. 48 (1983) 761. [14] A.F. Bogensch~tz and P. George, Legierungsueberzuege and Legierungsbaeder (Eugen G. Leutze Verl., Saulgau, Wuertt., 1975) p. 140.
[15] G.N. Belozerski, Phys. Star. Sol. (a)46 (1978) 131. [16] G. Le Caer, J.M. Dubois, H. Fisher, U. Gonser and H.G. Wagner, Nu¢l. Instr. and Meth. B5 (1984) 25. [17] K. Kanchev and S. Nikolov, to be published.
87
AMORPHOUS MAGNETIC STATE ESTABLISHED BY MOSSBAUER SPECTROSCOPY ON POWDERS OBTAINED BY BOROHYDRIDE REDUCTION Dimiter BUCHKOV, Serafim NIKOLOV *, Iovka DRAGIEVA and Mina SLAVCHEVA Higher Institute of Mechanical and Electrical Engineering * Higher Chemical Technological Institute, Sofia, Bulgaria Received 21 October 1985; in revised form 13 June 1986
The MiSssbauer spectra of a series of iron-cobalt-boron powders with different boron contents are discussed in association with the conditions of borohydride reduction. The most important structural pecufiarities due to the composition and the amorphous-crystalline state of the powders are established on the basis of the hyperfine field distribution.
1. Introduction Considerable attention has been paid lately to the peculiarities of fine metal and alloy powders prepared by reduction of dissolved metal salts with sodium borohydride. A series of papers [1-4] deals with the magnetic properties, particle size and shape, and corrosion resistance of these powders as well as with their dispersion and application in the magnetic recording media. The incorporation of boron as a result of the reduction process necessitates registering of chemical and structural short-range order in the microcrystalline and amorphous states of the powders. Our X-ray studies [5] showed these powders to be amorphous. We investigated further [6,7] the principal technological parameters of the reduction process, i.e. reducing agent concentration, kind and amount of the complex-forming agent, effect on the amount of boron incorporated in the powders of the addition of a second and a third metal to the main one belonging to the iron-nickel-cobalt group. Data concerning the particle size and shape and the effect of the magnetic field during the process on the texturing of the interglobular space are presented in ref. [8]. Precise investigations [9-12,6] on the phase composition and amorphous state of systems with a similar composition obtained by other methods demonstrated the specificity and possibilities of
MtSssbauer spectroscopy. A new method was proposed [13] for the estimation of the internal hyperfine magnetic field distribution P(H). The purpose of the present paper was to study, by this method, the disordered microcrystalline and amorphous states of powders obtained by borohydride reduction.
2. Sample preparation and experimental procedines The powder samples with the general composition (Fe,Co)l_xB~ were prepared by chemical reduction of aqueous solutions of metal salts with NaBH 4, different kinds and amounts of complex forming agents being used, the protection against oxidation being the same for all samples. Table 1 shows the initial concentrations of the reducing and complex-forming agents as well as the boron contents of the samples as determined quantitatively by a known method [14]. The powders were pelleted using polyvinyl alcohol as a binder and were pressed under a low pressure to be made suitable for MSssbauer studies. The thickness of the absorber with respect to the iron content was 20 mg/cm 2. The spectra were recorded according to a standard scheme of a MSssbauer spectrometer functioning with a constant acceleration, with a multichannel analyzer
0304-8853/86/$03.50 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
88
D. Buchkov et a L / M'6ssbauer spectroscopy on iron- cobalt-boron powders
Table 1 Technological conditions of the preparation of powdery samples by borohydride reduction Sample
Complexforming agent
Complexforming agent concentration (mol/I)
Reducing agent (NaBH 4) concentration (mol/1)
Boron content in the sample (wt~)
A
citric acid citric acid citric acid lactic acid lactic acid potassiumsodium tartrate
0.010
0.25
1.74
8.38
0.021
0.25
1.52
7.37
0.031
0.25
0.87
4.34
0.0690
0.062
2.51
11.48
0.0690
0.079
3.02
13.86
0.0035
0.189
5.25
22.27
0.069 0.0684
0.25
3.34
15.15
B C D E F
K
lactic acid glycerol
and a 57Co source on a palladium substrate. The width of the outer lines of a standard M/Sssbauer a-Fe thin absorber was below 0.3 mm/s. The spectra were obtained at room temperature (RT, 295 K) and at liquid nitrogen temperature (LNT, 77 K) and are presented with respect to an a-Fe MiSssbauer absorber. The spectrum of sample K immediately after its preparation as well as the spectrum K* of the sample upon its crystallization in vacuo at 10 -6 Torr and a temperature of 650 K for half an hour and subsequent pelleting are given in fig 3.
3. Results and discussion
The main technological conditions of preparation of the powders, which determine the composition and the structure of the final product, are the rate-determining factors such as concentration of reducing and complex-forming agents and ratio between the metal ions in the solution (table 1). The alteration of these parameters (the other technological conditions being constant) results in substantial changes in amount and arrangement of
Boron content in the sample (ate)
the iron, cobalt and boron atoms, which is evident from the MSssbauer spectra given in figs. 1 and 2. Fig. 1 presents the spectra of a series of samples (A, B, C) prepared with varying concentration of the complex-forming agent. The effect of the latter consists in a decrease in the rate of the process of powder formation with a increasing concentration of the complex-forming agent. The spectra of samples D, E and F given in fig. 2 show the effect of the different boron contents obtained by varying the reducing agent amounts and using different kinds of complex forming agents. The iron/cobalt ratio in all samples is 5.7:1. Fig. 3 shows the spectra of sample K as well as the MSssbauer spectrum of the same sample after its crystaniTation (spectrum K*). For all the samples investigated the spectra look the same at room temperature and nitrogen temperature. Therefore, the broadening of the lines is not due to superparamagnetic phenomena in the samples [15]. The spectrum K* of the crystallized sample shows narrower lines, which unambiguously indicates the transition of sample K from the amorphous to the crystalline state.
89
D. Buchkov et al. / M'6ssbauer spectroscopy on i r o n - cobalt-boron powders
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The spectra of all samples have two principal components: a sextuplet produced by a predominating ferromagnetic fraction in the sample and a paramagnetic doublet in the central part of the spectrum. The intensity ratio of these two comPonents varies depending on the preparation conditions of the samples. On the basis of the measured values of the isomeric shift, 8E = 0.4 m m / S and the quadrupole splitting 8 -- 0.5 m m / s , this superparamagnetic phase corresponds to an oxide phase containing Fe 3+. For K and K* sampies this oxide phase is below 2%. The properties of the samples greatly depend on the magnetic fraction which corresponds to the
Fig. 2. M ~ s b a u e r spectra at room temperature (RT) of samples D, E, F with different boron contents due to different NaBH 4 concentrations.
sextuplet component of the spectrum. This component is obtained as a result of overlapping of a large number of elementary sextets, each of them depending on a different value of the hyperfine magnetic field H at the iron nucleus. Due to the chemical and topological disorder of the shortrange surrounding, the hyperfine magnetic field with respect to the different iron atoms can be considerated as a random quantity characterized by a density of distribution P ( H ) . For the sake of brevity, the latter is called hyperfine field distribution. The other M/Sssbauer parameters (quadrupole splitting and isomer shift) in the case of amorphous and disordered ferromagnetic alloys have a considerably weaker effect on the shape of the spectrum. They cause an additional broadening of the lines and eventually the appearance
90
D. Buchkov et al. / M~ssbauer spectroscopy on i r o n - c o b a l t - b o r o n powders
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Fig. 3. Spectra of sample K at r o o m temperature (RT) and liquid nitrogen temperature (LNT), and of sample K* at L N T (after crystallization of sample K during thermal treatment).
of a slight spectrum asymmetry. As a first approximation, the latter factors can be neglected, especially in the case of a more general discussion of the magnetic structure and the P ( H ) distribution• It is difficult and not always possible to achieve a good accuracy in the determination of P ( H ) from the experimentally recorded spectrum. However, the approximate shape and principal parameters of I ' ( H ) can be obtained by different methods [16]. A new method [13] of computer decom-
position of the symmetric spectrum is used• This method can easily be adapted to the more complicated case of a paramagnetic component in the central part of the spectrum by truncation of calculating to this region. In addition, the slight assymetry due to a low (different from zero) mean value of the quadrupole splitting affecting the magnetic fraction is taken into account• For this purpose, the quadrupole shift of the four inner sextet lines during decomposition is pre-set. The final P ( H ) distribution is obtained after a special deconvolution procedure [17] of the resultant one-component spectrum eliminating the Lorentzian broadening of the lines• T h e P ( H ) distributions presented in fig. 4 for the three most illustrative cases indicate the different types of local short-range order in the powders under investigation• For the sake of convenience, the arrows in the upper part of this figure show the positions of the hyperfine magnetic field for crystalline alloys with analogous compositions obtained by different authors [9,12]• The Roman numerals I, II and III denote the H values given in the cited papers for the different crystallographic non-equivalent positions of the iron atoms which are typical of tetragonal structures, isostructures of the type FeaP (three positions) and ortho, rhombic structures of the type Ni3P (two positions) [12]. In the all three experimental cases presented in fig. 4, the right-hand side of the distribution shows single peak of Fe-Co metal phase with no boron among the nearest neighbours of iron. This phase exhibits the highest intensity (about 41%; of the magnetic fraction) for the crystallized sample K* and the lowest intensity (about 9%) for sample K. The hyperfine magnetic field obtained for this component is (362 + 3)kOe, which in the case of FessCo]5 is in good agreement wi~ the data reported for the system Fe-Co [10]. The hyperfine field distributions in fig. 4 demonstrate the quantitative changes in phase composition due to crystallization. The powders obtained at a high reduction rate display a disorder of the main component atoms (iron, cobalt, boron)• Upon heat-treatment, concentration gradients of the components appear within the limits of the separate particles. As a result, the amount of a
D. Buchkov et aL / M'6ssbauer spectroscopy on iron-cobalt-boron powders I
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I
I
I
I
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91
termination of the dominating form during crystallization is difficult. However, on the basis of the P ( H ) distribution (fig. 4, sample K) one may conclude that the ordering in the short-range surrounding of the iron atoms is not completely arbitrary, contrary to the case of the dense-random-packing model. Formations with 2,3 and 4 boron atoms as near neighbours of iron are prevailing according to the chemically correlated disorder model proposed by Vincze et al. [9] for F e - B metallic #asses. The P ( H ) distribution of sample B (fig. 4) illustrates the decrease in the rate of the powder synthesis due to the effect of certain complex-forming agent. The amount of the metal phase (crystalline, bcc F e - C o ) is considerably larger, but the total boron amount in this sample is smaller. In this case, formation of F e - B phases is observed on which type I-positions dominates over type II. This non-uniform population is not possible when crystal phases of the type FeaB are formed. Hence, during borohydride reduction process, the iron-boron phases formation begins as in the amorphous state.
References
400
t
HC,Oe3 Fig. 4. Hyperfine magnetic field distribution P(H) obtained from the spectra of samples K*, B and K.
crystalline bcc F e - C o phase increases. In powders not subjected to a heat treatment the presence of boron causes a large disorder in the short-range surrounding which cannot be interpreted as a simple superposition of microcrystalline formations, i.e. this is a ease of topological disorder characteristic of the amorphous state. Crystallites of the types (FeCo)3B and (FeCo)2B appear during crystallization. Due to overlapping of the lines for (FeCo)2B with those for Fe m- and Fen-positions in (FeCo)3B in the cases of tetrahedral and orthorhombic forms, respectively, a quantitative de-
[1] A. Watanabe, T. Uechori, S. Saitoh and I. Imaoka, IEEE Trans. Magn. MAG-17 (1981) 1455. [2] A. Baltz, W.O. Freitag, A.P. G-reiferand V. Suchodolski, J. Appl. Phys. 52 (1981) 2456. [3] A. Baltz and W. Doyle, J. Appl. Phys. 50 (1979) 780. [4] Y. Taketomi, A. Watanabe, Y. Tokuoka, H. Sugihara and Y. Imaoka, J. Magn. Magn. Mat. 31-34 (1983) 905. [5] I. Dragieva, G. Gawrilov, D. Buchkov and M. Slavcheva, J. Less-CommonMetals 67 (1979) 375. [6] D. Buchkov, S. Nikolov, I. Drag/eva and M. Slavcheva, Yubileina nauchna sesiya"20 godini VMEI-Varna",Varna (1983). [7] I. Dragieva, M. Slavchevaand D. Buchkov, J. Less-Common Metals 117 (1986) 311. [8] D. Mehandjiev, I. Dragieva and M. Slavcheva, J. Magn. Magn. Mat. 50 (1985) 205. [9] I. Vincze, T. Kem6ny and S. Arajs, Phys. Rev. B 21 (1980) 937. [10] I. Vincze, I.A. Campbell and A.J. Meyer, Solid State Commun. 15 (1974) 1495. [11] J.M. Dubois and G. Le Caer, Nucl. Instr. and Meth. 199 (1982) 307. [12] M. Wojcik, H. Lerchner, P. Deppe, F.S. Li and M. Rosenberg, J. Appl. Phys. 55 (1984) 2288.
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D. Buchkov et al. / M~ssbauer spectroscopy on iron- cobalt-boron powders
[13] S. Nikolov, Solid State Commun. 48 (1983) 761. [14] A.F. Bogensch~tz and P. George, Legierungsueberzuege and Legierungsbaeder (Eugen G. Leutze Verl., Saulgau, Wuertt., 1975) p. 140.
[15] G.N. Belozerski, Phys. Star. Sol. (a)46 (1978) 131. [16] G. Le Caer, J.M. Dubois, H. Fisher, U. Gonser and H.G. Wagner, Nu¢l. Instr. and Meth. B5 (1984) 25. [17] K. Kanchev and S. Nikolov, to be published.